a coil resists a change in current. Putting a reverse biased diode across the coil, as speedy said, allows the current to continue to flow when the current to the coil stops.
greg - Philadelphia & Reading / Reading
In my control days we called it the 'free-wheeling diode'. All inductive loads powered by DC got one right at the coil (relay coils, solenoids, etc.). 1N4007 was pretty common - we bought them by the reel and used them by the dozens, but any diode with PIV greater than the control voltage will work.
I see this is an old thread, but if the OP is still interested, afaik a reverse diode and/or a resistor in parallel with the coil would be sufficient.
The spark is generated because there is a EM field set up in the coil that suddenly has nowhere to go when the circuit is broken, so the voltage rapidly spikes until it is high enough to spark. All you need to do is provide an alternate path. A resistor in parallel with the coil would do that. Keep in mind that it will also increase the current load on whatever is powering the coil and pick your resistor accordingly, eg, resistor with the same resistance as your coil would double the current needed by the circuit.
Personally I use just a diode on inductive loads, placed so that the diode cathode is on the coil anode and diode anode is on the coil cathode (aka flyback diode or suppression diode). That way, when the coil is powered, the diode is reverse biased and no current flows through it. When the circuit is broken, the residual EM field is oscillating inside the coil, and can essentially be short circuited through the diode. Fast response Schottky diodes are best, eg, the 1N58xx series.
I don't think there's much need for a capacitor in terms of spark prevention, but it never hurts to have one or two on a load like a coil to moderate voltage drops and spikes when the coil is turned on and off. Just be sure to have a diode in place as well to protect the cap from being reverse biased, as some types of capacitors react unfavourably to that occuring.
I would have to look up very old notes from apprenticeship days (no longer kept) so as not to miss-lead you. But a Google search for "spark quench circuit" will get you results.
Rob
how are resisters and capacitors used to prevent arcing with inductive loads like uncoupling magnets.